A magnetic resonance imaging apparatus is an imaging apparatus configured to magnetically excite nuclear spin of a patient placed in a static magnetic field with an RF pulse having the Larmor frequency and to reconstruct an image based on magnetic resonance signals emitted from the patient due to the excitation.
A magnetic resonance imaging apparatus is different from an image diagnostic apparatus using radial rays such as X-rays in that a patient is not exposed to radiation. Thus, magnetic resonance imaging apparatuses have been increasingly widely used in clinical practice as an apparatus capable of diagnostic imaging in a less-invasive manner.
Further, since blood vessels can be distinguishably imaged by magnetic resonance imaging without using contrast agent, various imaging techniques of non-contrast MRA (Magnetic Resonance Angiography) have been studied. Specifically, various non-contrast imaging techniques to which the above basic imaging methods such as a TOF (Time Of Flight) method and a phase contrast method are applied have been developed. As one of such non-contrast MRA technique, a method of depicting blood vessels based on four-dimensional vector information obtained by ECG (electrocardiogram) synchronization scan of a magnetic resonance imaging has been developed. The above-described four-dimensional vector information includes three-dimensional image data and its time phase.
A bloodstream image obtained by the above-described non-contrast imaging is an image in which blood flow velocity and blood flow volume are visually depicted (in the present specification, hereinafter, a bloodstream image means a non-contrast bloodstream image unless otherwise specifically noted). In order to appropriately diagnose and remedy blood vessel disease such as arteriosclerosis, ischemic heart disease, and cerebral ischemia, it is important to obtain more accurate bloodstream information by analyzing a bloodstream image.
When data acquired by the above-described non-contrast magnetic resonance imaging are analyzed by a conventional medical image processing apparatus, sufficient information can be obtained for a blood vessel with a comparatively large vascular diameter such as an aorta and a pulmonary artery (hereinafter, such a blood vessel is referred to as a major blood vessel). However, it is difficult to obtain sufficient information for a blood vessels which has a comparatively small vascular diameter and is branched from a large blood vessel (hereinafter, such a blood vessel with a small vascular diameter is referred to as a branched blood vessel).
In non-contrast MRA using a phase contrast method as an example, a track of a blood flow is depicted by tracing flow-velocity vectors calculated for respective voxels. When a vascular diameter of a branched blood vessel is smaller than the voxel size, a flow-velocity vector of this branched blood vessel is not calculated at the position where this branched blood vessel branches from its major blood vessel. In general, one flow-velocity vector is calculated for one voxel and it is impossible to depict tracks of blood flows in both of a major blood vessel and its branched blood vessel. When spatial resolution is increased in order to depict such a branched blood vessel, this causes another problem that imaging time increases.
Moreover, when sufficient information on a branched blood vessel is not obtained in image diagnosis, a lesion region causing a symptom such as ischemia which appears on a patient cannot be identified in some cases.
As mentioned above, a branched blood vessel is connected to another blood vessel such as a major blood vessel. Accordingly, if sufficient information on a branched blood vessel is obtained, it becomes possible to predict and/or diagnose occurrence of blood vessel disease such as ischemic heart disease and cerebral ischemia. Additionally, if sufficient information on a branched blood vessel is obtained, it becomes possible to occlude the branched blood vessel by treatment using a medical instrument such as a stent so that supply of nutrition to an important organ is not interrupted.
Furthermore, a new blood vessel which has appeared due to neoangiogenesis caused by a malignant tumor is a factor of rapidly advancing malignant grade of the tumor. If such a new blood vessel can be observed, it leads to early detection of a tumor.